Studies of all residential water consumption uses suggest that approximately 40% of the total potable water supplied to a residential household is used for toilet flushing. Another 40% is used for bathing. Only ten percent has any real public health need of meeting potability quality criteria (i.e., used for direct drinking, food preparation, and food preparation materials washing).
The unique coincidence of these two magnitudes suggests that bathing water might well be adequate to serve the toilet flushing function, with no need of its retaining potable qualities. In addition, unlike kitchen waste water (which contains food residues, oils, etc.), or laundry waste water (which contains fabric lint and other particulates), bath water is only likely to contain small amounts of soap wastes and perhaps minor amounts of body oils. The same observation can be made of waste water from lavatories. By limiting the waste water to be recycled for toilet flushing purposes to only these two sources, their combined volumes approximate the amount of water volume needed to provide all toilet flushing quantities needed to supply the toilet flushing demands of the same number of household residents as generate the bath water and lavatory waste water volumes.
Concepts for reuse of sub-potable water have been extensively reported in the literature. Among them, systems involving residential recycling, generally referred to as "gray" water systems, make use of those portions of residential waste water which do not contain waste streams containing human body wastes (so-called "black" water). "Gray" water has been recycled for reuse for a variety of purposes for which potability is not required (i.e., lawn watering, car washing, etc.), but in all such applications, the "gray" water does not re-enter the building.
In the mixture of "gray" water, in addition to waste water from showers, lavatories, and bath tubs, the waste streams from the kitchen contain food wastes collected in the kitchen sink, dish washer, and garbage disposal (if present), and waste streams from laundry trays and clothes washers containing lint and materials collected on the clothes being washed, plus waste water resulting from floor mopping containing miscellaneous particulates, have been included. All such foreign materials mitigate against reuse of gray water within the household.
Distribution of sub-potable water on a community wide basis has also been proposed, but has generally been rejected on the basis of cost of constructing a second water distribution system. Also, there is a perceived risk of the possibility of connection to the wrong water main when potable water use is required.
Concurrently, in response to a growing recognition of developing shortages in potable water supply systems, efforts have been launched to further minimize demand for potable water being supplied to residential buildings by imposing maximum flow rates on faucets and shower heads, and maximum permissible volume per flush on the sizing of flush tanks and flush valves on urinals and water is closets.